Heart-rate tuned comb filters for processing photoplethysmogram (PPG) signals in pulse oximetry

Ludvik Alkhoury, Ji won Choi, Chizhong Wang, Arjun Rajasekar, Sayandeep Acharya, Sean Mahoney, Barry S. Shender, Leonid Hrebien, Moshe Kam

Research output: Contribution to journalArticlepeer-review

4 Scopus citations


Calculation of peripheral capillary oxygen saturation (SpO 2) levels in humans is often made with a pulse oximeter, using photoplethysmography (PPG) waveforms. However, measurements of PPG waveforms are susceptible to motion noise due to subject and sensor movements. In this study, we compare two SpO 2-level calculation techniques, and measure the effect of pre-filtering by a heart-rate tuned comb peak filter on their performance. These techniques are: (1) “Red over Infrared,” calculating the ratios of AC and DC components of the red and infrared PPG signals,(AC/DC)red(AC/DC)infrared, followed by the use of a calibration curve to determine the SpO 2 level Webster (in: Design of pulse oximeters, CRC Press, Boca Raton, 1997); and (2) a motion-resistant algorithm which uses the Discrete Saturation Transform (DST) (Goldman in J Clin Monit Comput 16:475–83, 2000). The DST algorithm isolates individual “saturation components” in the optical pathway, which allows separation of components corresponding to the SpO 2 level from components corresponding to noise and interference, including motion artifacts. The comparison we provide here (employing the two techniques with and without pre-filtering) addresses two aspects: (1) accuracy of the SpO 2 calculations; and (2) computational complexity. We used both synthetic data and experimental data collected from human subjects. The human subjects were tested at rest and while exercising; while exercising, their measurements were subject to the impacts of motion. Our main conclusion is that if an uninterrupted high-quality heart rate measurement is available, then the “Red over Infrared” approach preceded by a heart-rate tuned comb filter provides the preferred trade-off between SpO 2-level accuracy and computational complexity. A modest improvement in SpO 2 estimate accuracy at very low SNR environments may be achieved by switching to the pre-filtered DST-based algorithm (up to 6% improvement in SpO 2 level accuracy at −10 dB over unfiltered DST algorithm and the filtered “Red over Infrared” approach). However, this improvement comes at a significant computational cost.

Original languageEnglish (US)
Pages (from-to)797-813
Number of pages17
JournalJournal of Clinical Monitoring and Computing
Issue number4
StatePublished - Aug 2021

All Science Journal Classification (ASJC) codes

  • Health Informatics
  • Critical Care and Intensive Care Medicine
  • Anesthesiology and Pain Medicine


  • Comb filter
  • Electrocardiography (ECG)
  • Motion artifact
  • Peripheral capillary oxygen saturation (SpO )
  • Photoplethysmography (PPG)
  • Pulse oximeter
  • Pulse oximetry


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